Microstructural study of yttria stabilized zirconia buffered sapphire for YBa2Cu3O7−δthin filmsM. S. R. Rao, C. P. D’Souza, P. R. Apte, R. Pinto, L. C. Gupta, S. Srinivas, and A. K. Bhatnagar Citation: Journal of Applied Physics 79, 940 (1996); doi: 10.1063/1.360876 View online: http://dx.doi.org/10.1063/1.360876 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/79/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Epitaxial growth of YBa2Cu3O7−δ films on oxidized silicon with yttria and zirconiabased buffer layers J. Appl. Phys. 74, 3614 (1993); 10.1063/1.354500 Preparation of Pb(Zr0.54Ti0.46)O3 thin films on (100)Si using textured YBa2Cu3O7−δ and yttriastabilizedzirconia buffer layers by laser physical vapor deposition technique Appl. Phys. Lett. 63, 30 (1993); 10.1063/1.109740 YBa2Cu3O7 thin films grown on sapphire with epitaxial yttriastabilized zirconia buffer layers Appl. Phys. Lett. 61, 2412 (1992); 10.1063/1.108181 Epitaxial yttriastabilized zirconia on (1102)sapphire for YBa2Cu3O7−δ thin films Appl. Phys. Lett. 58, 304 (1991); 10.1063/1.104669 In situ single chamber laser processing of YBa2Cu3O7−δ superconducting thin films on Si(100) with yttriastabilized zirconia buffer layers Appl. Phys. Lett. 57, 1578 (1990); 10.1063/1.103358

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Microstructural study of yttria stabilized zirconia buffered sapphirefor YBa 2Cu3O72d thin films

M. S. R. Rao, C. P. D’Souza, P. R. Apte, R. Pinto, and L. C. GuptaTata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400 005, India

S. Srinivas and A. K. BhatnagarSchool of Physics, University of Hyderabad, Central University, P.O. Hyderabad 500 134, India

~Received 29 June 1995; accepted for publication 4 October 1995!

Yttria stabilized zirconia~YSZ! buffer layers were grown by rf-magnetron sputtering on ther ~11̄02!plane of sapphire for YBa2Cu3O72d ~YBCO! thin film deposition. Microstructural changes of YSZbuffer layers grown using different sputtering conditions~5, 10, and 20 mTorr; Ar/O2 gas mix ratioof 9:1 and 1:1! were monitored by atomic force microscopy~AFM!. Films grown using a loweroxygen partial pressure~5 mTorr! and a higher Ar/O2 ratio ~9:1! showed smooth surface morphologyand the average surface roughness increased with an increase in oxygen partial pressure. YBCOfilms in situ grown by pulsed laser deposition on sapphire with a YSZ buffer layer deposited usingoptimized sputtering parameters~5 mTorr gas pressure, 9:1 Ar/O2 ratio! yielded the highest criticaldensity,Jc'4.53106 A cm22 at 77 K. An excellent correlation between microstructure andJc hasbeen found and AFM has proved to be important for the study of the microstructure of films.© 1996 American Institute of Physics.@S0021-8979~96!07801-8#

I. INTRODUCTION

High-Tc superconductors are the most suitable candi-dates for applications in the area of passive microwave de-vices such as resonators, filters, delay lines, etc.1,2 Due to itslow dielectric constant~e8'6! and dielectric loss tangent~tand,1024!, as compared to that of other commonly usedsubstrates such as LaAlO3 ~LAO! and SrTiO3 ~STO!, sap-phire~a-Al2O3! is the most suitable substrate material for thegrowth of YBa2Cu3O72d ~YBCO! thin films for microwavedevice applications.1 However, YBCO cannot be directly de-posited on the sapphire substrate for the following two rea-sons:~1! There is a large lattice mismatch@'13% betweenYBCO ~a50.382 nm andb50.388 nm! and sapphire#. Thecrystal structure of sapphire is normally denoted by a simplerhexagonal unit cell3 ~primitive: rhombohedral! withaH50.476 nm andc'1.29 nm; and~2! aluminum diffusesfrom a-Al2O3 into the YBCO lattice during the film growthprocess at elevated temperatures~'750–800 °C!, which isdetrimental to superconducting transition temperature forzero resistance,Tc and critical current density,Jc .

In view of the importance of the sapphire substrate foruse in microwave device applications, it is desirable to over-come the above two problems. Yttria stabilized zirconia~YSZ! is a suitable material which can act as a good bufferlayer between sapphire and YBCO to prevent Al diffusionfrom the sapphire substrate. It also lattice matches consider-ably well with both sapphire and YBCO. YSZ@Y2O3!m~ZrO2!12m# has a cubic structure4 for m50.08–0.4~i.e., 8% Y2O3/92% ZrO2–40% Y2O3/60% ZrO2! and thelattice constant varies from 0.513 to 0.518 nm. The optimumvalue ofm is found to be 0.1~10% Y2O3/90% ZrO2!. The r~11̄02! plane of sapphire is preferable for the growth of~h00!oriented YSZ films.

Although a considerable amount of work has been doneon the growth of YBCO films on YSZ buffered sapphire5–8

and other buffer layers,9,10 the critical current densities ob-

tained so far have not been very high~1–23106 A cm22 at77 K!5,6 as compared to those realized on LAO and STOsubstrates~6–103106 A cm22 at 77 K!.11 Recent study onthe growth of Ag-doped YBCO films on unbuffered sapphireyielded aJc of 13106 A cm22.12 This discrepancy demon-strates the importance of controlling the growth conditionsand hence, the microstructure of YSZ films grown on a sap-phire substrate in order to realize high quality YBCO thinfilms. Even though oriented YSZ films could be obtained onsapphire single crystalline substrates, a well-controlledgrowth of YSZ films that result in an excellent microstruc-ture is a prerequisite for the growth of high quality YBCOfilms. In view of this, we have made a thorough and system-atic study on the effect of variation of growth conditions onthe microstructure of YSZ films suitable for YBCO filmgrowth. In this article, we present the effect of variations ingrowth conditions on the quality of YSZ films by rf magne-tron sputtering technique and the subsequent YBCO filmgrowth by pulsed laser deposition~PLD! on these buffer lay-ers. A detailed account of atomic force microscopy~AFM!study, which has proved to be a very useful tool in analyzingthe microstructure of the YSZ buffer layers and its bearingon the critical current density of the YBCO films, is alsopresented and discussed.

II. EXPERIMENT

The growth of YSZ buffer layers on a single-crystalliner~11̄02! plane of sapphire and subsequent YBCO depositionwas carried out using rf magnetron sputtering and PLDtechniques,13 respectively. The rf sputtering chamber waspumped using a CTI cryopump to give an oil-free environ-ment and a base vacuum of 131027 mTorr. A halogen lampheater assembly was used to heat substrates up to tempera-tures as high as 800 °C. Commercially obtained YSZ targets100 mm in diameter, 3–4 mm thick having a 99.9% puritywere used for sputtering. Ag-doped YBCO targets were fab-

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ricated for PLD using high purity starting chemicals ofY2O3, BaCO3, CuO, and Ag2O. Stoichiometric powderswere thoroughly mixed and the powder mix was calcined at900 °C for 20 h followed by regrinding and pelletization ofthe powder. Pressed pellets~15 mm diameter, 2–3 mm thick-ness! were sintered at 910 and at 500 °C for 20 and 40 h,respectively, in flowing oxygen and were slow cooled toroom temperature in oxygen ambient. The microstructuralinvestigation was carried out using x-ray diffraction~XRD!and AFM ~Digital Instruments, Nanoscope III!. Resistance–temperature~R–T! measurements on the superconductingfilms were carried out using a linear four-probe method usinga CTI cryocooler in the temperature range of 12–300 K.Critical current density measurements were carried out onpatterned 15-mm-wide microbridges of films.

III. RESULTS AND DISCUSSION

A. Buffer layer growth

This study was aimed at growing good quality YSZbuffer layers using different growth conditions. It is wellknown that high oxygen levels during the growth processdecrease the sputtering rate and the processing parametersbecome sensitive to high oxygen partial pressures presentduring sputtering. However, a partial pressure of oxygen isessential for growing films of oxide materials. Hence, formaintaining the required stoichiometry of the YSZ com-pound, it is desirable to use oxygen gas in appropriate ratioalong with the sputtering gas argon. We have, therefore, cho-sen three different sputtering pressure conditions viz. 5 , 10,and 20 mTorr and in each case two Ar/O2 ratios were usedviz. 9:1 and 1:1. Other parameters kept constant during thegrowth process were: 125 W rf power, 800 °C substrate tem-perature, and 4.5 cm target–substrate distance. In all thecases presputtering was done for 5 min and the film thick-ness was in the range of 60–70 nm.

Figure 1 shows the XRD of the YSZ buffer layers grownon a sapphire single-crystalline substrate using differentgrowth conditions. For films grown using a 9:1 Ar/O2 gasmix, XRD showed sharp~h00! reflections, which is indica-tive of good crystalline nature of the films. The full width athalf-maximum~FWHM! of ~200! reflection was 0.22°, 0.26°,and 0.30° for films grown using 5, 10, and 20 mTorr chamberpressures, respectively. On the other hand, for films grownusing a 1:1 Ar/O2 gas mix, XRD showed very low intensity~200! reflections and the FWHM is higher~0.28°, 0.32°, and0.45° for 5, 10, and 20 mTorr chamber pressures, respec-tively!. The higher angle reflections~400! were not seen inthis case. Thus, XRD showed that films grown using a cham-ber pressure of 5 mTorr and 9:1 Ar/O2 gas mix ratio showedbetter crystallinity as compared to those grown using otherconditions. The YSZ lattice parameter calculated from XRDstudy was found to vary from 0.513 to 0.516 nm when thesputtering pressure varied from 5 to 20 mTorr. This variationin the lattice parameter shows that there is a change in theYSZ composition for films grown using higher O2 partialpressure. This means in~Y2O3!m~ZrO2!12m, m tends to in-crease from an optimum value~0.08–0.1! to higher valueswhich is not good for the stability of the film. To corroborate

the XRD studies, we carried out detailed AFM studies tovisualize the microstructural details of the YSZ buffer layersgrown with different growth conditions.

B. Atomic force microscopy studies on YSZ bufferlayers

AFM is a simple but versatile technique that has beenfound to be extremely useful in visualizing the true micro-structure of the YSZ films. It has given a great deal of infor-mation for a very good understanding of the growth processof the buffer layers as well as the YBCO films with regard tovariations in growth parameters. Figure 2~a! shows the AFMimage of the YSZ buffer layer grown using 5 mTorr~Ar/O2ratio51:1! pressure, and Fig. 2~b! shows its three-dimensional ~3D! surface plot which clearly shows out-growths on the film surface with an average roughness~ar! of0.6 nm. In comparison, the microstructure of the films grownusing the same chamber pressure~5 mTorr! and a gas mixratio of 9:1 indicates very smooth surface morphology~ar50.3 nm! as shown in Fig. 2~c!. Figure 2~d! shows the 3Dsurface plot of the same. Similarly, outgrowths are seen onthe films grown using 10 mTorr~1:1! sputtering conditions~ar51 nm! as shown in Fig. 3~a!. However, although no suchoutgrowths are seen in Fig. 3~b! for films grown using 10mTorr ~9:1! growth conditions, the average film roughnesshas increased to 0.6 nm from that of 0.3 nm observed for 5mTorr ~9:1! film growth parameters. Figures 4~a! and 4~b!depict the surface morphology of films grown using a cham-ber pressure of 20 mTorr and an Ar/O2 gas mix ratio of 1:1.The outgrowths that have started to appear in the case offilms grown using chamber pressures 5 and 10 mTorr andAr/O2 gas mix ratio of 1:1 have increased to the extent that

FIG. 1. XRD traces of YSZ buffer layers grown onr-plane~11̄02! sapphireusing different growth conditions. Value in parenthesis indicates the Ar/O2

ratio.

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the surface morphology of the film appears granular~averagegrain size50.3 mm!. Grain boundaries separating out thegrains can be clearly seen. Some of the grains are oriented attilt angles with respect to neighboring grains and the averagesurface roughness is 5 nm. On the other hand, as seen inFigs. 4~c! and 4~d!, films grown using 20 mTorr~9:1! growthconditions have shown a surface roughness of 1 nm. Thus,the summary of the AFM study is as follows.~1! The averagefilm roughness increases with increase in chamber pressure~5 to 20 mTorr! for the Ar/O2 gas mix ratio of 9:1.~2! Forfilms grown using an Ar/O2 gas mix ratio of 1:1, a largenumber of outgrowths leading to an increase in surfaceroughness are seen. Thus, it is evident that the average sur-face roughness increases with increase in sputtering pressure~5, 10, and 20 mTorr!. This clearly implies that higher O2partial pressure during the growth process has a drastic effecton the microstructure of the films. An increase in O2 partialpressure increases the number of negative ions~O2, O2

2!generated in the plasma.14 These negative species are accel-erated away from the target in the cathode dark space towardthe substrate kept at a positive potential. Spatial variation inthe film composition results due to negative ion backsputter-ing, and hence, the quality of the film is affected as seen in

the present study. We observed that films deposited at higherO2 partial pressure have yielded films with slightly lessthickness~'60 nm! as compared to that~'70 nm! grownusing lower oxygen partial pressure. This shows that thesputtering rate is reduced with an increase in oxygen partialpressure. Films grown to thickness of the order of'100–200 nm resulted in extremely rough surfaces.

C. Growth and microstructure of YBCO films on YSZbuffered sapphire

YSZ buffered sapphire substrates grown using the abovesix different growth conditions were used for the depositionof YBCO thin films using PLD. It was shown earlier thataddition of 5 wt % Ag to a YBCO superconductor dramati-cally improved the microstructure,Jc and microwave surfaceresistance~Rs! compared to pure YBCO films.11,15 The 5wt % Ag added YBCO targets were used in this study and,we refer to this composition as YBCO-AG for simplicity. Thegrowth parameters used for realizing high quality YBCO-AGfilms on LAO were optimized earlier~Jc'83106 A cm22 at77 K!, and reported elsewhere.11 The following conditionswere used for PLD growth of YBCO-AG films on

FIG. 2. AFM images of YSZ buffer layers grown using a 5 mTorrchamber pressure and Ar/O2 gas mix ratios of 1:1 and 9:1;~a! 5 mTorr ~1:1!—outgrowthscan be seen and the average surface smoothness is 0.6 nm;~b! a 3D plot of the same;~c! 5 mTorr ~9:1!—smooth surface morphology with no outgrowths~average surface roughness'0.3 nm!; and ~d! a 3D plot of the same.

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the buffer layers: a 248 nm laser wavelength~UV, excimer!,600 mJ laser energy, 2 J cm22 laser fluence, 10 Hz repetitionrate, 5 cm target–substrate distance, and 750 °C substratetemperature.

To realize high quality superconducting films on a YSZbuffered sapphire substrate, it is very important that the sur-face morphology of the YSZ buffer layer is very smooth tofacilitate the growth of highly textured superconductingfilms. The fact that lattice parameters of YBCO~a50.382nm andb50.388 nm! and YSZ~ a50.513–0.518 nm! devi-ate quite strongly, good crystalline growth of films resultsonly when the diagonal of thea–b plane of the YBCO filmgrows along the YSZ axes, i.e., azimuthal rotation by 45° ofthe YBCO a–b plane with respect to the YSZa axis asshown in Fig. 5. Even in such a configuration, there exists alattice mismatch~'5%! between YBCO and YSZ as com-pared to substrates with lesser lattice mismatch viz., LAO~3%! and STO~1.2%!.

XRD has shownc-axis oriented YBCO film growth inall the cases. Figure 6 shows the XRD traces of YBCO filmsgrown in situ on sapphire with YSZ buffer grown using the

sputtering parameters: 10 mTorr~9:1! and 5 mTorr~1:1!. TheFWHM of ~005! reflection for 5 mTorr~9:1! is 0.30° whichis indicative of good crystallinity of the films. Similarly, theFWHM of ~005! reflections of the films grown on buffersusing 10 mTorr pressure are comparatively good~0.32°–0.40°!. On the other hand, YBCO film deposited on thebuffer grown using sputtering conditions of 20 mTorr~1:1!has shown the highest FWHM value of 0.48°. Further, themicrostructure differs markedly for YBCO films depositedon buffer layers grown using different growth parameters.Figures 7~a! and 7~b! show the microstructure of YBCO-AGfilms grown on the buffer layer with very smooth surfacemorphology realized using the growth conditions of 5 mTorrsputtering pressure and 9:1 gas mix ratio. The microstructureappears similar to that observed on the YBCO-AG filmsgrown on LAO substrates that yielded highJc . This is asexpected since the base film YSZ, which acquired smoothsurface morphology, must have facilitated easy alignment ofthe YBCOa–b axis diagonal with thea axis of YSZ. In asimilar way, YBCO-AG films deposited on buffer layersgrown using an Ar/O2 gas mix of 9:1 and chamber pressures10 mTorr @Fig. 7~c!# and 20 mTorr have resulted in compa-rable grain structure growth, although the average surfaceroughness in these cases is higher. However, films depositedon buffer layers grown with a higher oxygen ratio~1:1! haveshown no proper grain structure and the surface morphology,as can be seen in Fig. 7~d!, is very rough with the appearanceof undesirable voids on the surface.

D. Tc and Jc measurements

Although Tc has been found to be constant~'88 K! inall the cases, films deposited at higher oxygen ratios~1:1!have shown higherR300 K, large transition widthDTc'3–4K, and higher residual resistance~R0!, compared to that offilms deposited on buffer layers grown using lower oxygenratios ~9:1! as shown in Fig. 8. Further, the large differenceobserved in the microstructure has a direct correlation toJc .The highestJc'4.53106 A cm22 at 77 K has been realizedon YSZ buffered sapphire grown using 5 mTorr~9:1! sput-tering conditions. This is, by far, the highestJc reported onYBCO films deposited on YSZ buffered sapphiresubstrate.5,6 Jc’s ~the inset of Fig. 9! of the YBCO-AG filmsdeposited on buffer layers grown using other parameters areinferior compared to that realized on buffer layers grownwith optimized parameters@5 mTorr ~9:1!#. This shows thedirect correlation of microstructure andJc .

E. (Jc)1/2 vs (Tc2T) plots

The superconductor–normal metal–superconductor~SNS! model proposed by DeGennes16 and Clarke17 gives agood account of the quality of the superconducting filmswith respect to estimating the grain boundary widths whichlimit the Jc values. This model has been extensively used inunderstanding and judging the quality of the films in ourprevious studies.15,18The nature of the grain boundary sepa-rating the individual grains, in general, is the superconductoritself with inferior superconducting properties~e.g., oxygendeficient cuprate superconductors!. At temperatures a few

FIG. 3. AFM images of YSZ buffer layers grown using the sputtering con-ditions: 10 mTorr chamber pressure and Ar/O2 gas mix ratios of 1:1 and 9:1;~a! 10 mTorr ~1:1!—an increase in the number of outgrowths and averageroughness~ar'1 nm! is clearly seen;~b! 10 mTorr ~9:1!—the average sur-face roughness has increased to 0.6 nm compared to that grown at 5 mTorr~9:1!.

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FIG. 4. AFM images of YSZ buffer layer grown using a chamber pressure of 20 mTorr and Ar/O2 gas mix ratio 1:1 and 9:1;~a! 20 mTorr~1:1!—the surfacemorphology of the film appears granular with average surface roughness'5 nm ~average grain size 0.3mm!; ~b! a 3D plot of the same; some of the grainsare oriented at tilt angles with respect to neighboring grains;~c! 20 mTorr~9:1!—the increase in average surface roughness~'1 nm! is seen as compared tothat for films grown using low oxygen partial pressure~5 and 10 mTorr!; and ~d! a 3D plot of the same.

FIG. 5. YSZ has a cubic structure@a'0.514–0.518 nm#. Good crystallinegrowth of YBCO films results only when the diagonal of thea–b plane ofthe film grows along the YSZ axis; i.e., azimuthal rotation~by 45° of theYBCO a–b plane with respect to YSZa axis!.

FIG. 6. XRD traces of YBCO-AG films grown on YSZ buffers using thefollowing conditions: 10 mTorr~9:1! and 5 mTorr~1:1!. c-axis orientationcan be seen in both the cases.

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kelvin below the transition temperature of the fully super-conducting grains these grain boundaries act as metallic re-gions, thereby forming SNS junctions. However, at lowertemperatures these weakly superconducting grain boundaryregions do not offer much resistance to higher current flow asthey become fully superconducting. On the other hand, whenthe grain boundary volume is very small the current tunnel-ing by proximity effect improves and it is possible to realizehigh Jc values even at temperatures close toTc.

It is shown thatJc is proportional to (Tc2T)2 for SNStype junctions. NearTc , the equation that governs the SNStype junction is

Jc~T!}~Tc2T!2 exp~2aK21!, ~1!

or

AJc~T!}~Tc2T!exp@2~1/2!aK21#, ~2!

where a is the grain boundary width andK is the decaylength of Cooper pairs in the grain boundary~N! region.Considering the SNS type of coupling between the grains,the termK21 is given by

K21~5jN!5S \DN

2pKBTD 1/2, ~3!

whereDN is the diffusion coefficient andjN is the coherencelength in normal region.

At lower temperatures, whereN becomes fully supercon-ducting, K becomes large and higher currents can flowthrough the junction. A straight line plot ofAJc vs ~Tc2T)which goes through the origin shows that the coupling be-tween superconducting grains is SNS-like at temperatures afew kelvin below the transition temperature. The slope ofAJc vs (Tc2T! plot, which is an inverse exponential func-tion of ‘‘a’’ gives an estimate of the grain boundary width.

Figure 9 showsAJc vs (Tc2T) plots of YBCO-AG filmsdeposited on YSZ buffer layers grown using different sput-tering conditions. As can be seen, YBCO-AG films depositedon buffer layers grown using optimized parameters@5 mTorr~9:1!#, which have realized the highestJc, have given thehighest slope value as compared to others. An increase inslope indicates decrease in grain boundary width, which isalso evident from the microstructural study. A sys-

FIG. 7. AFM images of YBCO-AG films deposited on YSZ buffered sapphire grown at different conditions:~a! 5 mTorr ~9:1!—excellent microstructure ofYBCO-AG films grown on the YSZ buffer layer with smooth surface morphology. HighestJc'4.53106 A cm22 at 77 K has been realized;~b! a 3D plot ofthe same;~c! 10 mTorr ~9:1!—a comparable microstructure as that of the above, and~d! 5 mTorr ~1:1!—no proper grain structure with large voids andincreased surface roughness.

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tematic decrease in the slope values for films deposited onbuffer layers grown at different conditions shows the versa-tility of this model in estimating the film quality. These re-sults again corroborate very well with the microstructuralstudy.

IV. CONCLUSION

A detailed study on the growth of YSZ buffer layers onsapphire single-crystalline substrates by rf magnetron sput-tering has been made. AFM, which is a very versatile tech-nique, has been the main experimental tool used in visualiz-ing the true microstructure of the YSZ buffer layers grownusing different sputtering conditions. Films grown at loweroxygen partial pressure conditions~5 mTorr chamber pres-sure; Ar/O2 gas mix ratio59:1! resulted in smooth surfacemorphology suitable for YBCO film growth by PLD. ThehighestJc'4.53106 A cm22 at 77 K has been realized onYSZ buffered sapphire grown using the above optimizedbuffer layer growth conditions. An increase in oxygen partialpressure during the buffer layer growth resulted in an in-crease in the film roughness. YBCO films deposited on thesebuffer layers yielded much lowerJc values. Hence, realiza-tion of highly oriented YSZ buffer layers with very smoothsurface morphology is very important for the growth of highquality YBCO films on sapphire.

ACKNOWLEDGMENTS

The authors wish to thank Professor R. Vijayaraghavanfor discussions. Thanks are also due to S. P. Pai and S. C.Purandare for experimental assistance.

1G. C. Liang, R. S. Withers, B. F. Cole, S. M. Garrison, M. E. Johansson,W. S. Ruby, and W. G. Lyons, IEEE Trans. Appl. Supercon.3, 3037~1993!.

2S. Sridhar, D. H. Wu, and W. Kennedy, Phys. Rev. Lett.63, 1897~1989!.3A. S. Cooper, Acta Crystollgr.15, 578 ~1962!.4V. S. Stubican, R. C. Hink, and S. P. Ray, J. Am. Ceram. Soc.61, 17~1978!.

5Q. Li, O. Meyer, X. X. Xi, J. Greek, and G. Linker, Appl. Phys. Lett.55,1792 ~1989!.

6R. K. Singh, J. Narayan, A. K. Singh, and J. Krishnaswamy, Appl. Phys.Lett. 54, 2271~1989!.

7X. D. Wu, R. E. Muenchaunsen, N. S. Nogar, A. Pique, R. Edwards, B.Wilkens, T. S. Ravi, D. M. Hwang, and C. Y. Chen, Appl. Phys. Lett.58,304 ~1991!.

8K. Char, N. Newman, S. M. Garrison, R. W. Barton, R. C. Tabor, S. S.Laderman, and R. D. Jacowitz, Appl. Phys. Lett.57, 409 ~1990!.

9A. B. Berezin, C. W. Yuan, and A. L. de Lozanne, Appl. Phys. Lett.57, 90~1990!.

10T. Venkatesan, E. W. Chase, X. D. Wu, A. Inam, and C. C. Chang, Appl.Phys. Lett.53, 243 ~1988!.

11D. Kumar, M. Sharon, R. Pinto, P. R. Apte, S. P. Pai, S. C. Purandare, L.C. Gupta, and R. Vijayaraghavan, Appl. Phys. Lett.62, 3522~1993!.

12R. Pinto, P. R. Apte, M. S. Hegde, and D. Kumar, J. Appl. Phys.77, 4116~!995!.

13R. Pinto, S. P. Pai, C. P. D’Souza, L. C. Gupta, R. Vijayaraghavan, D.Kumar, and M. Sharon, Phys. C196, 264 ~1992!.

14S. M. Rossnagel and J. J. Cuomo, inThin Film Processing and Charac-terization of High Temperature Superconductors, AIP Conference Pro-ceedings No. 165, edited by J. M. E. Harper, R. J. Colton, and L. C.Feldman~AIP, New York, 1987!.

15R. Pinto, P. R. Apte, S. P. Pai, and D. Kumar, Phys. C207, 13 ~1993!.16P. G. DeGennes, Rev. Mod. Phys.36, 225 ~1964!.17J. Clarke, Proc. R. Soc. London Series A308, 447 ~1969!.18M. S. Ramachandra Rao, R. Pinto, S. C. Purandare, P. R. Apte, L. C.Gupta, R. Vijayaraghavan, B. S. Datar, and V. S. Shirodkar, Supercon. Sci.Technol.8, 108 ~1995!.

FIG. 8. R-T plot of YBCO-AG deposited on YSZ buffer grown using twodifferent conditions: 5 mTorr~9:1! and 20 mTorr~1:1!.

FIG. 9. ~Jc!1/2 vs Tc–T plots of YBCO-AG films grown on YSZ buffer

layers grown using different growth conditions:~a! 5 mTorr ~9:1!; ~a8!: 5mTorr ~1:1!; ~b! 10 mTorr ~9:1!; ~b8!: 10 mTorr ~1:1!; ~c! 20 mTorr ~9:1!;~c8!: 20 mTorr~1:1! YBCO-AG films deposited on YSZ buffer layers grownwith optimized conditions of 5 mTorr~9:1! have yielded the highestJc'4.53106 A cm22 at 77 K ~see the inset! and the highest slope as com-pared to others which means a much reduced grain boundary width. Thedashed line shows the plot of YBCO-AG film grown on LAO for compari-son.

946 J. Appl. Phys., Vol. 79, No. 2, 15 January 1996 Rao et al. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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